Monolithically integrated semiconductor tunable lasers are widely used in the telecommunications industry for transmitting optically modulated light along optical fibres. Commonly, in such applications, the optical signals of many lasers are wavelength division multiplexed (WDM) or densely wavelength division multiplexed (DWDM) with transmission on standardised transmission channels. Two principal telecommunications bands, namely the C Band (191.6-196.2 THz) and the L Band (186.4-191.6 THz), have standard wavelength channels defined by the International Telecommunications Union (ITU) at spacings of 100 GHz (0.8 nm), 50 GHz (0.4 nm), or 25 GHz (0.2 nm). As well as requiring stability in the transmission wavelength, such wavelength multiplexed systems require the transmitting lasers to have a narrow linewidth. Laser linewidth is particularly significant in coherent transmission systems, in which a laser is provided both in the transmitter and in the receiver of each transmission link.
Historically, simple single longitudinal mode lasers with short lasing cavities were widely deployed and capable of operation on only one channel or tunable across a small number of channels, with their operating wavelengths being thermally stabilised through control of the operating temperature of each laser. However, more recently, lasers that are widely wavelength tunable have found favour with network providers. U.S. Pat. No. 7,145,923 describes such a design of widely tunable laser.
The lasing cavities of widely tunable lasers require a pair of lengthy, tunable distributed Bragg reflector sections (DBRs), a gain section and a phase control section on a common waveguide, in order to operate on a single longitudinal cavity mode. The DBRs are provided by gratings within the optical waveguide of the laser, and are tuned to control the lasing wavelength of the laser cavity. However, these DBR sections increase the length of the laser cavity, which results in more closely spaced longitudinal modes of the laser cavity. Effective transmission of an optical signal requires uninterrupted transmission on a single, wavelength stabilised longitudinal mode with a high level of discrimination between the intensity of the dominant lasing mode and unwanted side modes. To provide a high level of side mode suppression (i.e. a high side mode suppression ratio, SMSR) it has been necessary to minimise the length of the optical cavity of the laser. To reduce electrical interference, narrow electrical isolation regions are provided between the control electrodes that are over adjacent sections of the laser. The length (along the waveguide) of such isolation regions is kept to a minimum, being no more than a few μm (i.e. significantly less than 20 μm).
The present inventors have identified that, disadvantageously, the requirement for short laser cavities results in the production of optical outputs from the lasers with significant linewidths, commonly having a linewidth dominated by the population of photons within the laser cavity and by the round trip time of the laser cavity.